This invention relates to flame spray powders, and more particularly relates to tungsten carbide-cobalt agglomerated powders utilizing cobalt nitrate as a binder, and also relates to a method for producing such powders.
Generally speaking, powder for use with flame spray coating equipment must have a narrow size distribution and must be relatively free flowing. (As used herein the term "flame spray" is meant to refer generically to both flame spray and plasma spray). The narrow size range is necessary if all particles are to be heated uniformly. The flow is to enable a uniform and controllable feed through the small diameter tubes and orifices of the equipment.
Tungsten carbide is commonly made by reacting tungsten powder or tungsten oxide with carbon at high temperatures. The result is a powder of average diameter less than about 10 micrometers and typically less than about 5 micrometers which has very poor flow. To achieve good flow, such powders must be agglomerated by one of several processes well known to the art. Such processes typically use an organic binder of some sort, such as paraffin or one of the many organic waxes, to hold the agglomerates together.
The organic binders have two main disadvantages. First, they must be removed prior to final processing or use of the powder or part made therefrom. Complete removal is difficult and time consuming. Second, the agglomerates are not very strong. When powders are blended or sifted they tend to deagglomerate, and to plug the sifting screens. Also, when stored in warm areas, the powder particles fuse together because of softening of the binders.
When working with molybdenum and tungsten powders, it was recently discovered that ammonium molybdate and ammonium tungstate added to an aqueous slurry of molybdenum or tungsten powder would act as a binder of the powder being agglomerated. When the slurry is spray dried, the ammonium molybdate or ammonium tungstate is well distributed throughout the interstices of the dried agglomerate and provides a strong bond for subsequent operations. A subsequent reduction reaction permits conversion of the ammonium salt to pure metal which still acts as a good binder because of surface-to-surface bonding promoted between the metal particles at the reduction and/or heat treatment temperature.
For hard, wear resistant surfaces, tungsten carbide, WC, is usually mixed intimately with about 4 to 20 weight percent of cobalt powder and flame sprayed to form a coating. Normally, the cobalt is agglomerated along with the WC as previously described prior to flame spraying.
Replacing the organic binders normally used in spray drying would be desirable for the reasons already stated. However, ammonium tungstate reacts with cobalt to form a gel or large particle size precipitate, which would hinder the spray drying operation. Ammonium complexes of WC and/or cobalt are either nonexistent, not readily available commercially or chemically unstable. Many other soluble salts, either tend to evolve large amounts of gases during the decomposition heat treatment (carbonates, oxylates and oxychlorides), or leave contaminating residues (sulfates, silicates and boron containing compounds such as borax or boric acid), or are corrosive (chlorides, oxychlorides).
Furthermore, any soluble salt used as a binder must be compatible with the processing of the tungsten carbide-cobalt powder, that is, it must be capable of being removed or decomposed without promoting substantial decarburization of the WC.